In this specification where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge, or otherwise constitutes prior art under the applicable statutory provisions; or is known to be relevant to an attempt to solve any problem with which this specification is concerned.
Biogas is typically a waste product from sources including anaerobic digestion of waste materials, such as waste water sludge, animal farm manure sewage and manure, landfill wastes, agrofood industry sludge, or any source that organic waste is able to break down in an environment that is substantially free of oxygen. The biogas produced by these activities typically contains 40-60% methane, 25% to 50% carbon dioxide, 0% to 10% nitrogen, 0% to 1% hydrogen, 0% to 3% sulfur, and 0% to 2% oxygen, all by volume, as well as an assortment of trace impurities that can include siloxane, chlorine, volatile organic compounds, and ammonia.
Since biogas is typically generated from organic matter, it can be considered a renewable form of energy which can be used as a fuel for internal combustion engines and boilers to generate electricity and heat. The biogases, however, contain noxious impurities, among which may include siloxanes, hydrogen sulfide and organic sulfurs. These impurities can be harmful to the environment and can cause damage to heat and power generation devices. For example, siloxanes present in biogas produce silicon dioxide in the biogas combustion process which can be deposited within heat and power devices causing damage to internal components such as engine pistons, spark plugs, and exhaust treatment devices. The deposition of silicon dioxide within these internal components can cause premature equipment breakdown and/or require more frequent maintenance or overhauls of heat and power generation devices. It is also possible in fuel cell systems that siloxanes can be deposited on downstream catalysts forming silicates that cause an abrasion to moving equipment and breakdown of catalysts or heat exchangers.
There are various methods currently used to remove siloxanes from biogas. One siloxane removal method is known as the temperature swing process (TSP). In this process, raw biogas enters into a dual vessel bed system, where adsorbents such as activated carbons (ACs), inorganics (silica and zeolites) or polymeric resins adsorbs siloxane molecules and other harmful volatile organic compounds (VOCs), effectively removing them from the biogas stream. The purified biogas can then be used as the fuel for a gas engine. This system uses a one system design. Optionally, the system may use an adjustable cycle to alternate between processing vessels, which are regularly purged with hot gas stream during continuous operation. In another embodiment the system can also use a single vessel design. This self-regeneration system ensures the continuous operation of the process. However, there are some major problems associated with the regeneration procedure. For example, the TSP typically uses ambient air as a source to regenerate the saturated adsorbents in a temperature swing process. However, because the TSP process requires ambient air to be electrically heated to 50 to 400° C. an additional power consumption ranging from 20 to 300 kilowatts may be required for the removal process to purify 1200 SCFM (standard cubic feet per minute) of biogas. In an embodiment, the additional power consumption can range from 20 to 100 kilowatts. Accordingly, there is a need for a siloxane removal system that eliminates or reduces the additional power consumption required to heat ambient air to 50° C. to 400° C. for TSP.